It is known that that ferromagnetic iron oxide useful for high density magnetic recording should possess a high coercive force. There are several techniques for increasing the coercivity of a ferromagnetic oxide powder. These include, for example, adding cobalt to acicular ferromagnetic oxide.
U.S. Pat. No. 4,188,302 teaches the use of an aqueous suspension of .gamma.-Fe.sub.2 O.sub.3 containing cobalt and iron ions to form a single mixed coating on a ferromagnetic oxide core. The coercivity of the coated particles can then be increased by heating the ferromagnetic oxide particle for a sustained period.
The properties of coercivity as well as remanent magnetism are important characteristics of magnetic tapes for they directly reflect the recording density which a magnetic tape can achieve. Ever increasing demands to improve the areal density of recorded and retrievable information have put a great challenge on the recording media as well as magnetic particulate manufacturers to design, produce, and use materials which are capable of increasing recording density without sacrificing other performance aspects such as magnetic stability. This burden is particularly acute when one considers that about 80-85% of the tonnage and material costs of magnetic recording media coating is attributable to the magnetic particulates.
As alluded to above, one principal method of particle modification to improve coercivity is adding cobalt to Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 O.sub.3 which results in substituting cobalt for iron ions in Fe.sub.3 O.sub.4 or by filling vacant lattice positions in .gamma.-Fe.sub.2 O.sub.3 with cobalt ions.
Prior art modification methods have employed the use of FeOOH, Fe.sub.3 O.sub.4, .gamma.-Fe.sub.3 O.sub.4, and partially reduced .gamma.-Fe.sub.2 O.sub.3 to serve as core materials. To generalize, the materials in particulate form are suspended in aqueous solutions of cobalt, iron, or other transition metal ions traditionally as divalent salts and, in some cases, with complexing and reducing agents. A chemical reaction is carried out by adding equal or greater than stoichiometric amounts of a base such as NaOH, KOH or NH.sub.4 OH at specified conditions of temperature, pressure, concentration of modifying metal salt solutions and pH for a specific period of time. The core materials so treated are then separated from the aqueous phase, dried, and usually subjected to a heat-treatment which is carried out in oxidizing, reducing, or inert environmental conditions at a specified temperature for a specified period of time. The extent of increase or enhancement of magnetic properties specifically in coercivity and specific saturation and remanent magnetization which is achieved depends primarily on (1) the amount of cobalt, (2) the amount of other metallic ions, if present, (3 ) conditions of modification of the recited treatment reactions, and (4) the post heat treatment program.
There are, however, certain disadvantages associated with the above-described particle modification methods. For example, there is a certain lack of chemical as well as processing flexibility. Cobalt, an extremely expensive element, is not used efficiently while post heat treatment steps cause magnetic instability and provide for gas-solid reactions which are difficult to control.
From the material user's standpoint, the prior art processes are very complicated and require extensive equipment and manpower to justify an in-house effort to produce and use high-performance materials. These methods lack the flexibility for direct incorporation of modified materials into the coating formulations resulting in an invariable need for drying and post-treatment steps. Lastly, precise control of FeO content and, hence, of saturation and remanent magnetization, which increases low frequency response and sensitivity of the recording media, is very difficult.
It is thus an object of the present invention to produce high-performance ferromagnetic oxides without the difficulties experienced by prior art methods.
It is yet another object of the present invention to convert low-performance ferromagnetic oxides into high-performance ferromagnetic oxides with increased coercivity without the need for post-treatment heating steps.
It is still another object of the present invention to produce high coercivity ferromagnetic iron oxides using lesser amounts of cobalts than utilized in prior inventions or previously practiced methods.
It is yet another object of the present invention to produce a high-performance ferromagnetic oxide particle whereby the divalent iron oxide (FeO) content and hence the saturation magnitization can be precisely controlled.
These and other objects of the present invention will be more fully appreciated when considering the following discussion.